postmodernism n.

word-forming element meaning "after," from Latin post "behind, after, afterward," from *pos-ti (source also of Arcadian pos, Doric poti "toward, to, near, close by;" Old Church Slavonic po "behind, after," pozdu "late;" Lithuanian pas "at, by"), from PIE *apo- (source also of Greek apo "from," Latin ab "away from" see apo-).

1737, "deviation from the ancient and classical manner" [Johnson, who calls it "a word invented by Swift"], from modern (adj.) + -ism. From 1830 as "modern ways and styles." As a movement in the arts (away from classical or traditional modes), from 1924.

I wish you would give orders against the corruption of English by those scribblers who send us over [to Ireland] their trash in prose and verse, with abominable curtailings and quaint modernisms. [Swift to Pope, July 23, 1737]

also post-modern, post modern, by 1919, in frequent use from 1949, from post- + modern. Of architecture from 1940s; specific sense in the arts emerged 1960s (see postmodernism).

But it has been only during the later decades of the modern era — during that time interval that might fairly be called the post-modern era — that this mechanistic conception of things has begun seriously to affect the current system of knowledge and belief; and it has not hitherto seriously taken effect except in technology and in the material sciences. [Thorstein Veblen, "The Vested Interests and the Common Man," 1919]

So much for the misapplied theory which has helped set the artist's nerves a-quiver and incited him to the extremes of post modern art, literary and other. [Wilson Follett, "Literature and Bad Nerves," Harper's, June 1921]